Unpolarized lasers are commonly used in a variety of applications, including instruments to optically detect and size particles down to 0.1 microns in diameter. These instruments measure the light scattered from particles as they pass through the laser beam. Determination of the size of the particle is determined by the amount of scattered light that is detected. The noise floor in these instruments comes from light scattered by the background molecular gas in the particle-laser interaction region (background scattering noise) and is of two types: a fundamental noise from the photon statistics (shot noise) present even in a perfect laser and technical background scattering noise from an imperfect laser source (technical background scattering). The background scattering noise reduces the sensitivity of the instrument. Shot noise is not reducible. To improve sensitivity, methods are here described to reduce the noise from technical background scattering.
In the highest-sensitivity system, the only source of noise would be the shot noise. However, there are other sources of noise (technical background scattering) that result in lowered sensitivity and therefore, result in an increase in the size of the smallest detectable particle. One of these additional sources of noise comes from laser amplitude fluctuations. These fluctuations appear as technical background scattering on the molecular-scattered light signal above the shot noise limit. One method used to reduce the noise from laser amplitude fluctuations is to monitor the laser output to determine the fluctuations in the laser amplitude and subtract these fluctuations from the scattered light signal (or in general, any other desired signal). This type of direct subtraction, however, does not work as well as expected in an unpolarized laser system. Unpolarized lasers, by definition, have continuously changing modes of polarization. Polarization mode fluctuations give rise to an additional noise term that cannot be directly subtracted. The noise caused by polarization mode fluctuations is not generally appreciated. It arises because the spatial pattern of the molecular-scattered light depends on light polarization, and the scattered light detection system collects only a finite solid angle. The net effect is a sensitivity of the detected scattered light that depends on the polarization of the laser light. The laser output monitor used in the simple amplitude noise subtraction method described above has a different sensitivity to polarization than the detector used for the scattered light, because the laser output monitor is effectively sampling a different spatial region than the scattered light detector. Typically, the output monitor light has no sensitivity to polarization. Therefore, if a direct subtraction method is used for technical background scattering, there is imperfect noise cancellation of laser amplitude fluctuations.
Some methods described to reduce noise in a laser system have been described. U.S. Pat. No. 4,798,465 (Knollenberg, Jan. 17, 1989) and continuation-in-part U.S. Pat. No. 4,893,928 (Knollenberg, Jan. 16, 1990) describe a particle detection device having background noise reduction. The noise reduction is achieved by use of a plurality of linear detectors, where each detector senses a portion of the optical path. The signals from the detectors are parallel processed to reduce the effect of background molecular scattering. U.S. Pat. No. 6,061,132 (Girvin, May 9, 2000) describes a particle counter having a dual detector array, wherein a detector in one array is used for noise cancellation, a detector in the other array is used to detect the signal from a particle, and the signals are subtracted to reduce the noise. U.S. Pat. No. 5,467,189 (Kreikebaum, Nov. 14, 1995) describes a particle sensor which subtracts background scattering signals from particle signals. U.S. Pat. No. 5,121,988 (Biesener, Jun. 16, 1992) describes a particle detector having monitoring of laser output power and adjustment of the current supplied to the laser to compensate. U.S. Pat. No. 6,414,754 (Johnson, Jul. 2, 2002) describes use of an ionic coloring agent on portions of the instrument to absorb stray light.
None of the described methods of noise reduction takes into account the effect of polarization mode fluctuations on technical background scattering. An improved method for canceling the laser amplitude noise in an unpolarized laser system is needed.